Wireless Local Area Networks(WLAN) can be used to replace wired Local Area Networks or as extensions to wired LAN infrastructures and are standardized for example in accordance with the IEEE 802.11 standards.
In a Wireless Local Area Network, beacon frames are used for enabling WLAN terminals to establish and maintain communications in an orderly fashion. The beacon frames are transmitted by the Access Points(AP) at regular intervals and include a frame header and a body with various information, i.a. a Service Set Identifier(SSID) identifying the name of a specific WLAN and a beacon interval specifying the time interval between two beacon transmissions. The beacon interval is a static, configurable parameter in an Access Point.
One important purpose of the beacon frames is to inform the WLAN terminals about the presence of an Access Point in the area and to generate traffic so that the WLAN terminals can measure the Received Signal Strength Indicator(RSSI). Typically, the Beacon frames provide the WLAN terminals with information in support for their decisions to perform handovers between different WLAN Access Points.
When a WLAN terminal is scanning for other Access Points in the area, it stops sending and receiving traffic, as changing the operating channel and waiting for beacon frames to be transmitted by the Access Points has to take place. As Access Points are not synchronized with each other, the beginnings of the beacon frames are not synchronized. In worst case, a WLAN terminal has to remain on a channel for the entire duration of a beacon interval, thereby consuming battery power and reducing throughput, in order to receive the beacon frame. Scanning can be done in a number of ways to reduce latency and to reduce unavoidable drop in throughput of wanted traffic. At least passive scan, active scan and hidden scan scanning modes exist.
A drawback of active scanning is that WLAN terminal generates traffic by transmitting special requests to nearby Access Points. If there are large number of active WLAN terminals with active scanning capability, the throughput available for useful user payload data is decreased. The scanning data available from active scanning is unicast, therefore applicable only to initiating WLAN terminal, whereas beacon frames that are used in passive scanning are usable for all terminals within the coverage area.
Today beacon frames also contain a load information that informs WLAN terminals currently connected to a specific Access Point or considering making a handover to that Access Point about the load situation. This information helps the WLAN terminals in making correct handover decision, in addition to the information from the comparisons of RSSI readings obtained by scanning, and thus ensures that WLAN traffic is divided more evenly between all Access Points in the area.
As mentioned above, the beacon interval is a fixed, configurable parameter. Typically, the beacon interval setting is not touched at all in the WLAN network installation phase, but the default value selected by the equipment supplier is used. If the beacon interval is long, maximum capacity in the Access Point is achieved. However, it will the take a long time for WLAN terminals to scan for Access Points in the area and to update RSSI and load information for already found Access Points. This obviously reduces terminal throughput and wastes battery. On the other hand, if the beacon interval in short, passive scanning performed by the WLAN terminals will be faster, but the overall capacity of the Access Point will be reduced.
Thus, there is a problem with that a particular fixed beacon interval in one situation will be regarded as too long, while the same beacon interval in another situation it will be too short. As shown above, both of these situations will have a negative impact on the perceived overall performance.
Moreover, with dual band WLAN Access Points and dual band WLAN terminals this problem is even more severe since the amount of channels to scan through will be higher than in the case with only single band Access Points and single band terminals.